There have been six confirmed travel-related cases of Zika virus disease,all among residents of Harris County,where Houston is located,the Texas Department of State Health Services said.

In medical literature,there has been only one case of Zika transmitted sexually and one case in which the virus was detected in semen.

Zika virus poses relatively little threat to adults,with the majority of people infected displaying no symptoms. But there is more concern for pregnant women,with the virus linked tomicrocephaly – a condition in which babies are born with an abnormally small head.